Our objective is to better understand the mechanical basis of blood flow in the human lung. This is clinically important. In this regard a thorough theoretical understanding with full experimental verification will be useful. Encouraged by the success of our analytical approach to pulmonary blood flow for cat we would like to extend the same approach to human lung. In this approach, a theory is built on (1) detailed description of the vascular geometry, (2) measured elasticity of the blood vessels, (3) rheology of blood in blood vessels, and (4) basic laws of physics and appropriate boundary conditions. Differential equations are obtained and solved either analytically or numerically. The solutions are compared with physiological experiments. The specific aims of this proposal are (a) To obtain a complete set of data on morphometry of human pulmonary arterial and venous trees. Silicone elastomer casts of human lungs are made, measured, counted, and analyzed. Data for the diameters, lengths, and branching ratios of vessels in successive orders as determined using the Strahler system are collected. (b) To obtain a complete set of data on vessel elasticity of human lungs in every generation. For pulmonary vessels with diameter greater than 100Mum, the x-ray projection method is used. For vessels of 10-100Mum in diameter, histological thick sections (200Mum) of silicone elastomer perfused lungs are measured. (c) By combining the above data with data on rheology of blood in blood vessels, we can make a theoretical analysis of pulmonary blood flow. In these calculations, the sheet flow theory is used for the pulmonary capillary blood flow and an analogous "fifth power law" is used for flow in the arteries and veins assuming that they behave like elastic tubes. The site of flow limitation in zone 2 condition is located at the junctions of capillaries and venules. Tethering effects of parenchyma are included wherein the pulmonary veins and venules do not collapse when the local blood pressure falls below airway pressure, whereas the capillaries do. The theory yields the pressure-flow relationship of the whole lung and a longitudinal pressure distribution. (d) A critical check on the theory is important, and for this purpose several physiological experiments are made: (1) measurement of pulmonary vascular resistance, (2) flow limitation phenomenon under zone 2 condition. The performed and existing experimental results are used to evaluate the above theory.